Hi Dennis,
I am sorry for the delay of this reply.
Regarding your issue, look like i've found the culprit to this issue. Somehow, there are two different url for one same blog post from the feed. E.g.
https://www.haufe.de/immobilien/wohnungswirtschaft/gww-vorteile-von-passivhaeusern-bisher-nicht-erke...
http://www.haufe.de/immobilien/wohnungswirtschaft/gww-vorteile-von-passivhaeusern-bisher-nicht-erke...
I not sure why this is happening but look like there are two crons running and one is pointing to HTTP (with http) feed url and another one is pointing to SSL (with https) feed url. In EasyBlog feed importer, it threat these two urls as two different blog post hence when importing the feed post, you get duplicate items
I've added fix on your site so that the checking will ignore the HTTP or SSL to prevent duplicate item being imported. Below is the file I've modified on your site.
JOOMLA/administrator/components/com_easyblog/includes/feeds/feeds.php and added below code at line 176:
$uid = ltrim($uid, 'https://');
$uid = ltrim($uid, 'http://');
The above code should fix your duplicate items issue
Also, I've remove the duplicate item from your EasyBlog
Hope this help and have a nice day!